This invention relates to a semiconductor device having metal films formed on semiconductor regions or layers of different conductive types and a method of manufacturing the same. Herein, it is to be noted throughout the instant specification that such semiconductor regions or layers will be collectively referred to as conductive layers.
Recently, it is required in a semiconductor device such as a MOS transistor to reduce a parasitic resistance of an electrode such as a source electrode, a drain electrode, and a gate electrode of the MOS transistor with a reduction of a semiconductor device in size.
To this end, a proposal has been made for a method of selectively forming a metal film of, for example, tungsten having a relatively low resistance over a silicon substrate in a self-alignment manner through a chemical vapor deposition (CVD). However, where the tungsten films are formed on conductive layers having different conductive types by the above selective growth method, it has been pointed out that the tungsten films on the conductive layers have different thicknesses from each other. This is because a growth rate of the tungsten is variable in dependency upon the conductive types of the conductive layers.
Specifically, when the tungsten films are formed on an n-type conductive layer and a p-type conductive layer, respectively, the thickness of the tungsten film on the n-type conductive layer is different from that of the tungsten film on the p-type conductive layer, since the growth rates of the tungsten are different from each other on both the n-type conductive layer and the p-type conductive layer. Such a difference between thicknesses of the tungsten films causes a leak current and the like. To solve the problem resulting from the difference between the thicknesses, a conventional method is disclosed in Japanese Unexamined Patent Publication No. Hei. 7-193026 (namely, 193026/1995).
In the above conventional method, the conductive layers of different conductive types are doped with impurities of the same conductive type. Consequently, the surface of the conductive layers is transferred to the same conductive type. As a result, the tungsten films are deposited to the same thickness on the conductive layers of the different conductive types because the growth rate of the tungsten is kept invariable on the same conductive type.
However, additional an impurity layer remains between the conductive layer and the tungsten film in this conventional method. The additional impurity layer forms a PN junction with the conductive layer. Therefore, a desired device structure can not be obtained because of the additional impurity layer. According to the conventional method, the tungsten reacts with the silicon (the conductive layer) by heating to form a tungsten silicide. The additional impurity region is absorbed in the tungsten silicide, and thereby the additional impurity region vanishes.
Thus, the additional process is needed to eliminate the additional impurity layer in the conventional method. Further, such a tungsten silicide increases the parasitic resistance of the electrode. The high resistance of the electrode is undesirable for the semiconductor device.